The Purple Nightshade Blossoms can be found at Grimvale during the Full Moon Event. There you will found 3 purple Nightshade that can be harvest. So, you can get 6 flowers each hour.
To reach Grimvale, travel to Edron and walk to Stonehome. To the north of Stonehome is a ferry run by Cornell, as the picture below shows:
Ask him to sail you to Grimvale (Hi - Sail - Yes / Hi - Grimvale - Yes). Once there just follow the following way:
Anyway when isn't Full Moon on those places you can found Purple Nightshade in place of the Blooming Purple Nightshades. Attempting to harvest a non-blooming Purple Nightshade will display a server message:
This purple nightshade is currently not blooming. No blossoms can be harvested.